The Greenest Tree

What kind of tree should I plant in my back yard to soak up the most carbon?

My husband and I intend to plant some trees on our property this spring. We'd like to do our part in the fight against climate change, so we're looking for trees that can sequester exceptionally large amounts of carbon. Can you recommend a specific species, one that's the acknowledged champ at reducing greenhouse gases?

The Lantern wishes he could just name a single species that will meet your needs, thereby making this his easiest (and shortest) column. But alas, there's no one-size-fits-all answer to your query; the type of tree that's best for the environment will depend on your geographic location and its attendant climate, as well as the composition of the soil on your property. And to maximize your results, you'll have to commit to taking extra-special care of your trees during their formative years—and consider turning them into chairs or tables before they start to decay.

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The first part of the sequestration equation is a no-brainer: The bigger a tree, the more room it has to store carbon. According to the United States Forest Service, the best trees for carbon sequestration are those with large trunk diameters and dense wood. It also helps if the trees sport leaves in lieu of needles; choose a hardwood over a conifer.

Just as important as maximum size and leaf type, however, is rate of growth—the faster a tree grows, the greater the percentage of its life that's spent as an arborous titan. According to the Energy Information Administration, a 30-year-old hardwood that's classified as a speedy grower—a red mulberry, for example, or a laurel oak—will sequester an average of 69.5 pounds of carbon per year, versus 36.8 pounds for a hardwood whose growth rate is deemed only moderate, and a measly 16.8 pounds for a true slowpoke.

Unfortunately, bigger hardwoods often grow at a snail's pace, especially in northern climes where there's a dearth of sunlight and chilly winters to contend with. As such, the conventional wisdom has long been that trees planted in equatorial rainforests are much better at stemming global warming than those planted in, say, Alberta or Vermont—the logic being that warm temperatures and heavy rainfall will result in faster average growth.

But North American trees can be goosed to grow more rapidly, a task usually accomplished through the careful application of nitrogen-based fertilizers. The downside to this method, however, is that the use of such fertilizers results in increased emissions of nitrous oxide, a potent greenhouse gas. A cleaner approach may be to plant a mix of trees—some that are good at sequestering carbon, others that enrich the soil by depositing nitrogen. Studies (PDF) on eucalyptus plantations, for example, have shown that more carbon is sequestered overall when nitrogen-fixing mimosa trees are added to the land.

If you're planting a non-native species, your rate of growth may be lackluster no matter how much nitrogen you add to the soil. Do some research and find out which large, leafy, fast-growing hardwoods are native to your area. Native trees are also more likely to reach maturity and thrive for years.

Yet even the hardiest native trees are doomed to die someday, and in doing so, spew their carbon back into the atmosphere. (That's particularly bad news when the trees are killed as part of a timber company's clear-cutting efforts, since no young trees are left behind to help mitigate the losses.) If you're around to witness your trees' twilight years, consider keeping the carbon in place by turning them into furniture or building lumber, rather than letting them go gently into that good night.

OK, so let's say you don't have the inclination to research the perfect tree for your locale, nor the time to spend ensuring that your soil is suitably flush with nitrogen. If that's the case with you and your husband, you probably can't go too terribly wrong with a few of the following trees: yellow poplars, scarlet oaks, London planes, or American sweetgums. All are fast-growing hardwoods that require little maintenance (and thus little use of gas-guzzling equipment), and have proved to be solid carbon absorbers in tests. According to a 2002 survey of several hundred New York City trees, a yellow poplar (also known as the tulip tree) was the carbon sequestration champion, socking away an impressive 137.26 pounds of carbon. (The runner-up, strangely, was a European beech, at 112.39 pounds.)

The Lantern's answer might be different a few years hence, thanks to a recent wave of research on hybrid trees. Scientists in Asia and the United States are all working to breed trees that are far better at absorbing carbon than their predecessors; this past October, for example, Japanese researchers announced they'd developed a hybrid larch that fixes 30 percent more carbon than normal larches.

How might planting, say, a half-dozen such supertrees on your property affect your household's annual carbon footprint? Let's say that a hybrid yellow poplar can be engineered to sequester 30 percent more carbon than the regular variety, for a grand total of 178.44 pounds per year. Multiply that by six trees, and you've offset 1,071 pounds of carbon. That, in turn, translates into 3,931 pounds of carbon dioxide, or 1.78 metric tons. Estimates vary widely, but the average American household's annual carbon footprint is around 22 metric tons.

Of course, if you have enough property to plant six yellow poplars in the first place, your carbon footprint may be a lot higher than that.

Is there an environmental quandary that's been keeping you up at night? Send it to ask.the.lantern@gmail.com, and check this space every Tuesday.